The temperature of a comfort zone within a building can be controlled by modulating the airflow rate of warm or cool air supplied to the zone. This is typically accomplished with the use of at least two VAV valves, one for a warm air supply duct and another for a cool air supply duct. Both supply air ducts serve the same zone and may share a common blower. Downstream of the blower(s), however, the ducts convey separate airflows corresponding to separate heat exchangers, one for heating and the other for cooling.
The rate of airflow through each valve is modulated to meet the comfort zone's temperature conditioning demand as determined by a thermostate. At low demand, both valves may be substantially closed, and the speed of the supply air blowers may be reduced to save energy. However, it is usually desirable to maintain at least a minimum airflow rate for ventilation purposes, even though the temperature conditioning demand has been satisfied. When the temperature of the zone has reached its set point, many controls provide minimum airflow by slightly opening both the heating and the cooling valves an equal amount. When a demand for heat or cooling arises, the valves begin to open further or close accordingly. As the demand further increases, the valves continue to move, and eventually one fully closes and only the other is controlled to meet the demand.
Accurate modulation of airflow can be difficult when operating in a relatively narrow minimum airflow region, where both valves are only partially open a slight amount around the set point temperature. This is because accurate measurement of low airflow rates requires a relatively sensitive flow sensor. In addition, a slight change in valve position can cause a dramatic change in airflow at low airflow rates. A further complication arises when the heated air supplies more heat than an equal amount of cooled air can remove, or vice versa. This imbalance should be compensated to avoid a net heating or cooling effect when the temperature conditioning demand has been satisfied, i.e., the zone temperature equals its set point. In addition, the mixing of heated and cooled air should be avoided whenever possible to minimize energy consumption.